It is known that a photovoltaic semiconductor p-n junction can convert to electricity only that portion of the incident photon energy spectrum, typically solar radiation, which creates hole-electron pairs within a given semiconductor material. For example, in a silicon photovoltaic cell only that portion of the solar spectrum with energy in the vicinity of the 1.1 electron volts per photon and which exceeds the band gap energy of silicon is converted into electricity. Photons of lesser energy do not generate electricity. More energetic photons are strongly absorbed but much of the energy is lost in heating the cell, which heat can degrade the cell's energy conversion efficiency. To maximize the efficiency of a given photovoltaic cell, it is advantageous to convert as much of the available light as possible into an energy range to which such cell can respond in the generation of electricity before the light strikes the cell's surface.
One technique for achieving such conversion takes advantage of the fact that light falling upon a luminescent agent is characteristically reradiated or emitted in a narrow band of wavelengths of known energy content. Also, light absorbed by such an agent in one direction is reradiated in random directions. Such agents include, for example, pigments such as metal oxides and organic dyes which are used in scintillation counters, lasers, and the like. For the purpose of this invention the term "luminescent agent" includes all types of luminescent agents exhibiting all species of luminescence, including, but not limited to, fluorescence and phosphorescence.
It has been shown that the dispersal of a luminescent agent within an internally reflective sheet of transparent glass or plastic, one of whose major surfaces is exposed to light, concentrates and focuses a flux of light of known energy level toward one or more of the thin upstanding edge faces of the sheet. If a photovoltaic cell responsive to light at that energy level is optically coupled to such edge face, the energy conversion efficiency of the cell increases several times. In this invention, a light transmissive member of such construction and properties is termed a "luminescent member" and a photovoltaic solar collector employing such a member is termed a "luminescent solar collector". A luminescent solar collector of this type is fully and completely disclosed in Optics, Vol. 15, No. 10, pages 2299-2300, dated October, 1976, the disclosure of which is incorporated herein by reference.
This invention addresses the difficulty and expense of finding a luminescent agent or agents which have all of the desired properties for a solar collector application. The optimum luminescent agent absorbs over a very broad range of visible light and emits at a wavelength at or just slightly shorter than the band gap of the photovoltaic cell employed. It is important to the efficiency of the solar device that the luminescent agent not absorb the emitted light at all, or at least very little. It is difficult to find a luminescent agent which covers a broad part of the light spectrum, so in practice a plurality of luminescent agents have been employed, as in U.S. Pat. No. 3,912,931. It is also difficult to find a luminescent agent that does not absorb, at least to some extent, in the same light region where that agent also emits light. That is to say the agent reabsorbs light emitted by another particle of the same agent, and this results in scattering of the emitted radiation more than necessary or desired and the loss of some of this scattered radiation out of the collector or conversion into heat.
When a plurality of luminescent agents are employed in the same collector, the situation becomes even more complicated because it then becomes even more difficult to select groups of agents which interact in just the desired way. Further, beyond the optical requirements, all luminescent agents employed have to be compatible with one another and the materials employed in the member of the collector which contains those agents. For example, the light transmitting member which contains the luminescent agent or agents must not degrade in sunlight so as to interfere with the operation of the device, such as by developing cracks or crazing which will divert the incident light, render the device less transparent, form peroxides or other decomposition materials which could be harmful to the luminescent agents themselves, and the like.
Accordingly, it is exceedingly difficult to find an overall combination of a number of materials which are optimum for solar collector requirements and still stand up to exposure to sunlight and other weather elements for a matter of years, even decades, without undue degradation. Even if such a combination of materials could be found, they most likely would be very expensive and might even have other objectionable features not yet known.
However, there are known luminescent agents whose properties and peculiarities are sufficently understood that they can be employed successfully in a solar collector, but these agents may not be as stable as desired and, therefore, do not maintain their efficiency over the large period of years desirable for solar collector installation. Generally, the known luminescent agents will diminish totally or at least in part in their ability to function in the manner desired for solar collectors. Some dyes are known to have a half life in sunlight of only a matter of weeks while others may last longer, e.g., one or more years, but they all lose some efficiency or even approach cessation of operation over long periods of time, such as ten years or more, and this causes a gradual and continuing decrease in the efficiency of the solar collector in its ability to generate electricity. Thus, it is very desirable to have a means by which this decreased efficiency is corrected.
Replacement of the entire solar collector is possible, but this is quite expensive and troublesome since such installations can be quite large. It is also wasteful because the photovoltaic cells themselves and other parts of the solar collector assembly have much longer useful lives than the luminescent agents themselves.
This invention allows for regeneration of the efficiency of the solar collector notwithstanding the decrease in efficiency with time of the original luminescent agents in the collector. This invention also allows for continued use of the original photovoltaic cells and other non or lesser deteriorating members of the assembly.